Bioreactors are fermenters in which biological conversions of substances are performed with enzymes, microorganisms (bacteria, fungi, yeast, algae) as well as animal and plant cells. It should be possible to create optimum conditions, specific to each process, with respect to temperature, pH value, and nutrient concentration in the bioreactor. The tasks of a bioreactor therefore include mass transfer within the fluid phase (mixing), dispersion in a second phase—in most cases air—in order to obtain a large phase boundary surface for good mass transfer (separation), and thermal transfer for dissipating the generated heat. The design of a bioreactor depends on its applications and therefore must take into account the specific requirements of the biological system used.
In order to cultivate phototrophic microorganisms, so-called photobioreactors are used. Among potential designs, the airlift photobioreactor is considered to be especially suitable for cultivating phototrophic microorganisms and growing them to a high cell density. The airlift photobioreactor often has a column-shaped reactor vessel, in which introduction of air creates a fluid circulation within a loop established by the construction. The airlift bioreactor is divided thus into a gassed and ungassed zone, which are interconnected at the bottom and top so that the hydrostatic pressure differential causes a pump effect that results in an outflow of fluid in the gassed zone. As the mixing of the reactor medium is due exclusively to the aeration, this photobioreactor design enables good mixing and a high gas-fluid mass exchange with low energy consumption. Such an airlift photobioreactor is described, for example, in DE 199 16 597.
As cultivation of phototrophic microorganisms requires high-intensity incident light radiation, especially deep in the reactor, photobioreactors preferably have a large surface-to-volume ratio and thus require a relatively high expenditure of material. Because of the materials used, such as glass or Plexiglas, the production of such a photobioreactor is relatively complicated, material- and cost-intensive.